Loudspeaker having plastic diaphragm with compliance grooves

ABSTRACT

Sound reproducing equipment comprising a frame having a diaphragm supported at its peripheral edges by said frame. The frame and diaphragm are preferably formed of materials having acoustically compatible properties and the diaphragm is also preferably mounted on the frame in a manner to permit movement of the diaphragm as a complete entity at low frequencies. At the same time the diaphragm is provided with compliance grooves or corrugations containing a compliant material serving to reduce or eliminate &#39;&#39;&#39;&#39;break-up&#39;&#39;&#39;&#39; or the development of nodal points within the diaphragm. The diaphragm further may be provided with a high frequency propagator which cooperates with the diaphragm to increase the range of the vibrations produced by the equipment without introducing feedback or distortional characteristics during operation.

PATENTED MAR 91% 0 LW 0 MM QM NO m. m5 W a J w v E D H AT ORNEYS LOUDSPEAKER HAVING PLASTIQ DIAPHRAGM WITI-I COMPLIANCE GROOVES RELATED PATENT APPLICATIONS This application is a continuation-in-part of applicants copending application Ser. No. 342,827 filed Feb. 24, 1965 now issued as Pat. No. 3,351,719.

BACKGROUND OF THE INVENTION The present invention is directed to improved loudspeakers and other sound reproducing equipment which are of thin construction and light in weight and which are capable of use under widely varying temperature and humidity conditions, and in equipment and locations presenting unusual requirements in these respects. At the sametime the construction of the diaphragm and its mounting are such as to permit the faithful reproduction of sounds over a range of frequencies extending from, say 40 to 20,000 c.p.s. witha minimum of distortion.

The greatly increased use and application of loudspeaking and sound reproducing equipment, and the vast improvements in the fidelity of stereo and audio equipment has resulted in a demand for devices capable of meeting critical acoustical specifications while imposing limitations such asreduced size, weight and thickness, or unusual conditions of operation such as wide ranges in temperature and severe conditions of humidity. f

While certain of these requirements can be met by the use of special and unusual compositions or constructions, the advantagesproduced in any one or more of these respects are generally attained at the expense of other important characteristics. Thus it has been suggested that sound reproducing equipment may be provided with a relatively thin flat diaphragm instead of the more usual cone type of loudspeaker. Typical patents calling for such constructions are U.S. Pat. Nos. 1,498,384; 1,746,289; 1,802,826; 2,047,367; and 3,236,958. Such diaphragms have in general been formed of wood or other-relatively heavy materials, and while they are of particular advantage in the faithful reproduction of lowfrequency sounds or as woofers, their weight and inertia, and the power required to actuate the same generally precludes their use in light weight equipment, and instruments designed for reproducing high-frequency sounds and notes in the upper range of musical tones. It is therefore necessary in such equipment to provide a separately mounted and energized high frequency device or tweeter in order to reproduce a relatively wide range 'of sound frequencies.

In the alternative, it has been suggested that loudspeakers may be provided with stiff, light weight, diaphragms formed of cellular plastics materials as exemplified by U.S. Pat. Nos. 1,824,664; 2,905,260 and 3,046,362 as wellas British Pat. No. 510,707. However, when such stiff, light weight diaphragms are formed in a substantially flat shape, it is found that they have a marked tendency to undergo midand high-frequency breakup" due to vibration of the central and peripheral portions of the diaphragm at different frequencies or out of phase with due other, creating intermediate nodal points and zones within the diaphragm which result in sound distortions.

Furthermore, in practically all sound reproducing equipment heretofore produced there has been a marked disparity between the characteristics and acoustical properties of the diaphragm on the one hand, and those'of the frame and elements by which the diaphragm and magnetic elements associated with the voice coil are mounted on the other. As a result the latter elements may set up vibrations of their own independently of the diaphragm and voice coil when the equipment is actuated. As a result there is a tendency for interfering vibrations to be fed back to elements of the loudspeaker equipment so as to create poor frequency response, distortion, buzzing, and parasitic noises which impare the quality of the sounds produced.

i the diaphragm having a preferred SUMMARY or THE INVENTION In accordance with the present invention these objections or limitations of prior loudspeaker assemblies are reduced or eliminated and thin, light weight, sound reproducing equipment is provided which is capable of use under widely varying and adverse conditions of operation. At the same time the quality of the tones or sounds produced is materially improved and rendered consistent over a range of frequencies which may extend from 40 to 20,000 c.p.s. or greater.

THE DRAWINGS FIG. I is a perspective illustrating a typical form of sound reproducing assembly embodying the present invention;

FIG. 2 is a front elevation of the frame of the equipment illustrated in FIG. 1 with the diaphragm removed therefrom;

FIG. 3 is a'transverse sectional view through the equipment illustrated in FIG. I take on the line 3-3 thereof;

FIG. 4 is a rear view of the assembly illustrated in FIG. 1;

FIG. 5 is a sectional view through aportion of the frame and diaphragm taken onithe line 5-5 of FIG. 4;

FIG. 6 is a rear elevation of the diaphragm embodied in the sound reproducing equipment of FIGS. 1 to 5;

FIG. 7 is an enlarged sectional view of a portion of the diaphragm illustrated in FIG. 6, taken on the line 7-7 thereof; and i I FIG. 8 is a sectional view through the voice coil form and type of high frequency propagator carried thereby.

PREFERRED EMBODIMENT OF THE INVENTION In that form of the invention chosen for purposes of illustration in the figures of the drawings, the sound reproducing equipment embodies a flat marginal frame 2, which as shown, is rectangular in shape and provided with side members 4, 6, 8 and 10. The inner edges of the side members of the frame 2 are provided with inwardly projecting diaphragm supporting ledges 12 which, as shown in FIGS. 2 and 3, lie in a common plane facing the front 14 of the frame, as seen in FIG. I. The marginal portions 16 of a thin, flat, stiff and light weight diaphragm 18 are supported on the ledges l2 and surrounded by shoulders 20 which are of a height approximately equaling the thickness of the diaphragm. Inclined surfaces 22 extend outwardly from the upper edges of the shoulders 20 to the front 14 of the frame so as to present an outwardly flaring hornlike surface about the edges of the diaphragm 18.

As. shown in FIGS. 3 and 5 the frame 2 has an outer edge orplace in a suitable mounting means (not shown) and the assembly may be conceaied by the cloth or fabric covers or by any other suitable or decorative material through which the sounds produced may penetrate without distortion. In the alternative the unit may be utilized as an element or panel in a ceiling, wall, furniture or elsewhere as'desired.

A tubular voice coil form 24 is secured to the flat diaphragm 18 adjacent the center thereof and projects toward the opposite or rear face 26 of the frame 2 as shown in FIG. 3.

A magnet supporting rib 28 has the opposite ends thereof secured to opposite sides of the frame 2, such as the sides 4 and 8, and has a magnetic assembly 30 carried thereby and presenting an air gap 31 surrounding the voice coil from 24 and the voice coil 32 which is mounted on the form 24.

The magnet supporting rib 28 is preferably also formed of material whichis the same or similar in its acoustical or physical properties with the frame and diaphragm I8. Thus, the frame 2, diaphragm I8 and magnet supporting rib 28 may all be formed of expanded cellular plastic material or other acoustical compatible plastic material such as a solidified foam or expanded beads of polystyrene. An alternative conintegral and unitary molded assembly, which may be produced by injection molding or otherwise and composed of unexpanded plastic material having a density ranging from about 0.8 to 10.0 pounds per cubic foot. On the other hand the diaphragm 18 may be formed or stiff, light weight plastic material of a similar nature but having an expanded or cellular form. A typical composition adopted for use in forming the diaphragm is expanded polystyrene, preferably having a density ranging from about 0.8 to 1.8 pounds per cubic foot.

While the density of the material used in forming the frame and magnet supporting rib may be somewhat higher than that used in forming the diaphragm, and in some instances may, as indicated above, be formed of unexpa'nded or non-cellular material, it is most desirable to employ materials in forming the frame and magnet support which are sufficiently similar or compatible with the material used in forming the diaphragm so that the major elements of the entire assembly will have matching acoustical and physical characteristics presenting a minimum tendency to set up their own or independent and interfering vibrations or to undergo substantially different changes when subjected to widely differing conditions of temperature, humidity and the like.

The magnet supporting rib 28 is secured to the frame 2 adjacent the or or face 26 of the frame 2 and is spaced from and extends parallel to the diaphragm 18 as shown in FIG. 3. The magnetic assembly 30 is fixedly secured to the magnet supporting rib 28 and held in fixed position thereby. Further, as

shown in FIG. 3, the magnetic assembly 30 is preferably molded in place within the supporting rib 28 in position to surround the voice coil 32 carried by the voice coil form 24 on diaphragm 18. The assembly 30 includes a permanent magnet 34 which may be formed of a magnetic alloy such as Alnico or a ceramic material such as lndox" or the like. The magnet 34 is provided with a rear plate 36 and a central pole piece 38 both of which are formed of ferromagnetic material. A front plate or pole piece 40 is also formed of ferromagnetic material and contacts the permanent magnet 34. A central opening 42 in the pole piece 40 surrounds the voice coil 32 on the voice coil form 24. The sides of the central opening 42 in pole piece 40 are spaced from the central pole piece 38 so as to provide the air gap 31 of limited cross section in which the voice coil 32 of the diaphragm is located, as indicated in FIG. 8.

In the preferred construction the exposed faces of the rear and front plates 36 and 40 are substantially flush with the front and rear surfaces of the magnet supporting rib 28 whereas the permanent magnet 34 is larger in diameter than said plates so rib 28 but is also fixedly secured in place with respect to the axis of the voice coil 32. Moreover, even though the devices embodying the present invention are subjected to widely varying conditions of temperature and humidity the elements will be retained in the same accurately'disposed relation wit respect to each other by reason of the similarity or identity of the materials employed in forming the frame 2 magnet supporting rib 28 and diaphragm 18.

in accordance with the present invention, the diaphragm 18, as indicated above is preferably formed of stiff, light weight, expanded polystyrene, or other cellular plastic material, and in most instances has a thickness of about one-sixteenth to one-half inch depending upon the size and type of the sound reproducing equipment embodying the present invention. Further in order to permit the equipment to produce a wide range of sounds, varying in frequency from say 40 to 20,000 c.p.s., it is desirable to construct and mount the diaphragm in a novel manner which serves to reduce or eliminate the tendency to produce undesired break-up" or nodal lines, zones or points within the diaphragm during vibration thereof.

For this purpose the diaphragm 18 is preferably provided with compliance rings, lines, or corrugations which are located between the centrally located voice coil form 32 and the periphery or marginal edges 16 of the diaphragm. Furthermore it is desirable to secure the marginal edges 16 of the diaphragm to the supporting ledges 12 of the frame 2 in a manner to enable the entire diaphragm to serve as a sound generating piston capable of reproducing tones or vibrations in the lowest audible or subaudible range.

As illustrated in the drawings these advantages are attained by providing sufficient clearance 33 between the outer edges of the marginal portions 16 of the diaphragm and the'surrounding shoulder 20 of the frame to allow the diaphragm to flex without restraint being imposed on the edges of the diaphragm by said shoulders. Moreover, the means or material employed for securing the marginal portions 16 of the diaphragm to the ledges 12 of the frame are preferably of a yieldable or flexible nature. For this purpose an adhesive such as a silicone rubber or other strong but yieldable cement 48 has been found to be particularly suitable.

The compliance rings, lines or corrugations formed in the diaphragm 18 may be variously spaced and arranged. As shown in the drawings it has been found desirable in some constructions to provide compliance means in the form of concentric grooves 50 and 52 in the front or outer face of the diaphragm 18 and to provide the rear face of the diaphragm with a plurality of grooves which differ in diameter from the grooves 50 and 52 so as to be spaced radially therefrom. As shown in FIGS. 3 and 6, the grooves 54 and 56 on the rear face of the diaphragm 18 are positioned adjacent and on opposite sides of the inner or smaller diameter groove 50 in the front face of the diaphragm whereas the grooves 58 and 60 in the rear face of the diaphragm are similarly located adjacent and on opposite sides of the outer or larger diameter groove 52 in the front face of the diaphragm. In practice the inner groove 50 and the associated grooves 54 and 56 on the opposite side of the diaphragm 18 are somewhat greater in width than the outer groove 52 and its companion grooves 58 and 60. The depth of the grooves 50 and 52 in the front of the diaphragm may also be slightly less than the depth of the grooves in the rear face of the diaphragm. However, these dimensional characteristics of the grooves and the diameter thereof may be varied if desired. Nevertheless, it is generally preferred to employ an inner groove 50 which is so spaced radially from the voice coil form 24 as to enclose the area equal to about 50 percent or more of the total area of the diaphragm 18.

It will further be noted from FIGS. 1 and 6 that the outer groove 52 on the front of the diaphragm 18 may extend substantially to the edge of the diaphragm, whereas the associate groove 60 on the inner surface of the diaphragm may terminate short of the edge of the diaphragm so as to be essentially arcuate in form. Moreover the positions of the front groove 52 which are close to the edges of the diaphragm may in fact be extended laterally as indicated at 61 and be filled with yieldable material 65 to offset or prevent marginal influence on the functioning of the diaphragm during vibration.

The provision of such compliance grooves serves to allow the diaphragm to respond to, and faithfully reproduce, tones and sounds which vary materially in frequency, whereby the tendency for nodal points or interfering or out-of-phase vibrations to develop in different portions of the diaphragm is materially reduced. However, there is a tendency for such a fully complaint diaphragm to lose its integrity or unitary character so as to present substantially uncoupled zones whereby low-frequency vibrations may not be faithfully reproduced or may be distorted. In accordance with the present invention it is therefore desirable to insert a yieldable elastic composition, such as a silicone rubber material, into the grooves 50 and 52 in the front face of the diaphragm as indicated at 62. This material serves to impose a limited restraint between the zones of the diaphragm at opposite sides of the compliance means. If desired similar yieldable or elastic material may be inserted in the rear grooves of the diaphragm also. Such elastic or yieldable material accordingly serves as a filter or functions as a limited coupling means between the adjacent zones of the diaphragm which precludes undesirable or excessive breaknup, and independent or completely isolated vibration of the zones or elements of the diaphragm. As a result more uniform and accurate response of the complete diaphragm during vibration at low frequencies is attainable. By way of illustration, it is found in practice that a loss in response may develop in the range of about 600 c.p.s. If the yieldable restraining insert 61 in the compliance grooves 50 and 52 in the front of the diaphragm is omitted, whereas more uniform response is attained by reason of the limited coupling provided in accordance with the present invention.

It has further been found that the piston action of the full diaphragm, when it is rectangular in shape as illustrated in the drawings, is improved, and distortion may be reduced by providing relief means or zones of freedom adjacent the mid portions of the marginal edges 16 of the diaphragm. For this purpose, as shown in FIGS. 1, 3 and 5, the diaphragm supporting ledges 12 of the side numbers 4, 6, 8 and 10 of the frame 2 are cut away or molded so as to be of lesser height than the remainder of said ledges 12 as shown at 94, 96, 98 and 100 respectively. Nevertheless limited points or dabs of less yieldable bonding material may be located at the central portions of the zones of freedom 94, 96, 98 and 100 as indicated at 102. In this way the marginal portions 16 adjacent the mid portions of the edges of the diaphragm 2 are characterized by limited but restrained freedom of movement which serves to prevent distortion or rubbing and buzzing. In addition, these bonds serve to stabilize the positioning of the whole diaphragm by minimizing the effects of cold flow of the bonding cements 48. As shown in FIG. 5, the relief means or zones of freedom 94, 96, 98 and 100 also afford air release passages about the edges of the diaphragm which serve to diminish the damping action of the air adjacent the edges of the diaphragm during vibration thereof.

In order further to extend the range and fidelity of sound reproducing equipment embodying the present invention it is also desirable in many instances to provide the device with a highfrequency propagating means. For this purpose the voice coil form 24, which is tubular in shape, may be formed of plastic coated paper, light metal such as aluminum, or other suitable material, and is secured to the diaphragm 18 by molding, cementing or otherwise attaching it fixedly and accurately in place adjacent the center of the diaphragm. The portion of the diaphragm surrounding the voice coil form is tapered inwardly and downwardly as shown at 66 so as to present an outwardly facing horn effect and at the same time allow the forward extremity 68 of the voice coil form to project in front of the adjacent portions of the diaphragm.

The high-frequency propagator is indicated generally at 70 and, as shown, includes an element 72 which presents a convex marginal surface 74 which is cemented or otherwise secured at its edges to the projecting extremity 68 of the voice coil form 24. The center of the element 72 is provided with a depression 76 surrounded by the vibratory element 78. The vibratory element is in the form of a thin metal ring having an inwardly and downwardly inclined attaching rim 80 cemented or otherwise fixed at its inner edge to the convex surface 74 of the element 72. The outer free marginal edge 84 of the vibratory element 78 is inclined toward the surface of the diaphragm and over the adjacent hornlike surface 66 so as to be spaced therefrom. In this way the high-frequency propagator is actuated directly and positively by the voice coil form in response to undamped vibrations induced by the voice coil and magnet assembly whereby the frequency response in the reproduction of tones of high pitch is materially improved. At the same time the coned surface 66 of the diaphragm directly adjacent the vibrating free edge 84 of the propagator aids in directing such vibrations forwardly with the lower frequency vibrations created by the piston action of the diaphragm as a whole. i

In this way the entire spectrum of frequencies, from the lowest tones to the highest pitch sounds, are faithfully reproduced by the various elements of the combination whereas only the minimum amplitude of piston motion is required. Accordingly both electrical and mechanical distortion are substantially reduced and increased electromagnetic efficiency is attained despite the unusually wide range of frequencies and response resulting upon operation of the equipment.

In order further to reduce the development of undesired induced or electromagnetic interference, the lead wires 104 through which current is supplied tothe voice coil 32 are mounted in fixed positions with respect to each other so that they cannot move relative to each other during vibration of the diaphragm. For this purpose a terminal strip or connecting element 106 is mounted on the frame 2 and preferably is embedded in the material of which frame'is formed during the molding operation. As shown in FIG. 2 the lead wires 104 may themselves be embedded or molded into the material used in forming the diaphragm '18. However, as shown in FIG. 5 and 7, it is generally preferable to provide grooves 108 in the diaphragm and to place the lead wires 104 in these grooves, after which the lead may be covered and embedded in a material such as the silicone rubber composition 110 which is inserted into the grooves 108 so as to substantially fill the grooves.

An additional feature which has been discovered in connection with the use of stiff, flat diaphragms produced by the molding of cellular polystyrene, such as expanded polystyrene beads, resides in the fact that the quality of the tone produced is improved when the surface of the diaphragm 18 is treated to remove the skin or continuous surface produced as an incident to the molding thereof. It is therefore desirable to remove such skin by desolving or otherwise roughening the surface of the diaphragm. This is preferably accomplished by applying acetone or other solventagent thereto and in a typical operation a solvent containing a dye or pigment may be applied to the diaphragm so as to simultaneously remove the skin" and color the diaphragm so that it will be invisible or blend with the color of any thin fabric or decorative covering material applied over the assembly in any installation to conceal the sound reproducing elements of the construction.

While the constructions shown and described are typical of sound reproducing equipment embodying the present invention, it will be apparent that the frame and diaphragm may be circular or elliptical in shape and may be constructed in various other ways in order to adapt the invention to thin, light weight equipment having other or special applications. In view thereof it should be understood that the particular devices shown in the drawings and described above are intended to be illustrative only and are not intended to limit the scope of the invention.

Iclairn:

1. Sound reproducing equipment embodying a diaphragm having a voice coil form carried thereby and located adjacent the center of the diaphragm, compliance grooves in said diaphragm surrounding said voice coil form and spaced therefrom to provide zones of different frequency resonance, and yieldably elastic material located in at least a portion of said compliance grooves between said zones of the diaphragm.

2. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is formed of stiff cellular plastic material.

3. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is formed of expanded polystyrene.

4. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is supported adjacent its edges by a frame having inwardly facing ledges engaged by the edgesof the diaphragm, there being air channels in said ledges at spaced points about said frame and between said frame and the adjacent surface of said diaphragm.

5. Sound reproducing equipment embodying a substantially flat diaphragm and a frame, said diaphragm being formed of molded cellular polystyrene and having a voice coil form secured to the diaphragm adjacent the center of the diaphragm, a plurality of compliance grooves formed in opposite faces of said diaphragm and spaced from said voice coil form, said compliance grooves being spaced unequal distances from said voice coil form and at least a portion of said compliance grooves having yieldable elastic material therein substantially filling said grooves, said frame having an inwardly facing ledge thereon engaged by'the marginal portions of said diaphragm, air channels formed in said ledges adjacent said diaphragm, and means securing said diaphragm to said frame.

6. Sound reproducing equipment embodying a diaphragm formed of stiff, light weight, cellular material, a voice coil form secured to said diaphragm adjacent the center of the 

1. Sound reproducing equipment embodying a diaphragm having a voice coil form carried thereby and located adjacent the center of the diaphragm, compliance grooves in said diaphragm surrounding said voice coil form and spaced therefrom to provide zones of different frequency resonance, and yieldably elastic material located in at least a portion of said compliance grooves between said zones of the diaphragm.
 2. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is formed of stiff cellular plastic material.
 3. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is formed of expanded polystyrene.
 4. Sound reproducing equipment as defined in claim 1, wherein said diaphragm is supported adjacent its edges by a frame having inwardly facing ledges engaged by the edges of the diaphragm, there being air channels in said ledges at spaced points about said frame and between said frame and the adjacent surface of said diaphragm.
 5. Sound reproducing equipment embodying a substantially flat diaphragm and a frame, said diaphragm being formed of molded cellular polystyrene and having a voice coil form secured to the diaphragm adjacent the center of the diaphragm, a plurality of compliance grooves formed in opposite faces of said diaphragm and spaced from said voice coil form, said compliance grooves being spaced unequal distances from said voice coil form and at least a portion of said compliance grooves having yieldable elastic material therein substantially filling said grooves, said frame having an inwardly facing ledge thereon engaged by the marginal portions of said diaphragm, air channels formed in said ledges adjacent said diaphragm, and means securing said diaphragm to said frame.
 6. Sound reproducing equipment embodying a diaphragm formed of stiff, light weight, cellular material, a voice coil form secured to said diaphragm adjacent the center of the diaphragm, a voice coil on said voice coil form, and electrical leads connected to said voice coil and extending to points adjacent the edge of the diaphragm, said electrical leads being secured to said diaphragm throughout the major portion of the length thereof, and located in grooves in one face of the diaphragm and held in place in said grooves by elastic material.
 7. Sound reproducing equipment as defined in claim 5, wherein the means for securing the diaphragm to the frame consists of a silicone rubber composition. 